Programming Forums > Application Development > C++

Ricky205 · May 19th, 2005, 12:38 PM

Hey, Im learning about global variables and i have a question concerning the example code below. Now I know that count is declared outside any function therefore can be used by any, but the value of count is given in the int main() function, so when it calls the second function how is it that it still works ? or is it that it doesnt matter where the value given is located as long as the variable is declared globaly or as long as there isnt a local variable with the same name that will interfere ?

thanks in advanced

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#include <iostream>
using namespace std;

void func1();

int count; // this is a global variable 

int main()
{
  int i; // this is a local variable 

  for(i=0; i<10; i++) {
    count = i * 2;
    func1();
  }

  return 0;
}

void func1()
{
  cout << "count: " << count; // access global count
  cout << '\n'; // output a newline
}

Ooble · May 19th, 2005, 1:31 PM

That will work fine, but yes, you're right - you can't declare a local variable if you have a global variable with the same name.

Ricky205 · May 19th, 2005, 2:06 PM

ok thanks , now I got a second question about it ..
what if u were to declare a global variable named var for instance. Now lets say you have 2 functions .. can u use var in both functions with each function having a different value for it .. say :

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outside all functions:
int var;

inside the first function :
var = 10;

inside the second function :
var = 50;

mackenga · May 19th, 2005, 4:33 PM

Yes, variables in different functions don't clash with each other.

If you have a global variable of a given name, for example foo, you can create a local one with the same name if you please.

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int foo;

int bar() {
  int foo;
}

Code inside the function, that is in the same scope as the local variable foo, will refer to the local variable when it refers to the variable called foo. Code outside it, in other functions, will refer to the global foo, unless they define a variable called foo of their own.

--
I just realised I kind of missed the point of what you were asking there. If you declared the global variable var, you could use it in two different functions, and they'd both be referring to the same variable. So in func1, if you set var to 10, then call func2, func2 will see the value 10 in it. If it now sets it to 20 and returns, and you then call func1, func1 sees the new value.

A global variable is one name and one storage location and it's shared throughout the program. It can only take one value at a time, and any function that assigns a value to the global variable will change the value any other function sees.

Berto · May 20th, 2005, 3:31 AM

I am not sure about c++ but as a rule of thumb when i am using java i tend to use this.<variable> for gloadls and then just the name of the variable for the local one.

mackenga · May 20th, 2005, 3:14 PM

There aren't any global variables in Java. As well as not being inside a function, global variables (in languages that have them) are not inside anything else.

Berto · May 21st, 2005, 10:25 AM

well its close enough to a global variable.

Aphex_Twin · May 21st, 2005, 11:12 AM

You can use the same name over and over in functions and blocks. C/C++ has a means of hiding the hierarchically higher variables.
For instance:

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int a;

void f () { int a; { int a; { int a; } } }

Is valid code. We have four diferent instances of the same variables (or four diferent variables with the same name). In each case the program sees only the most recently declared version. The global variable is accessible by the resolution operator (i.e. ::a). Though you cannot acces hierarchically higher local variables (such as the second "a" from the third block).

Global versions of variables can be accessed through ::my_var where my_var is the name of the variable.

May 19th, 2005, 12:38 PM	#1
Ricky205 Newbie Join Date: May 2005 Posts: 4 Rep Power: 0	C++ Global Variables question Hey, Im learning about global variables and i have a question concerning the example code below. Now I know that count is declared outside any function therefore can be used by any, but the value of count is given in the int main() function, so when it calls the second function how is it that it still works ? or is it that it doesnt matter where the value given is located as long as the variable is declared globaly or as long as there isnt a local variable with the same name that will interfere ? thanks in advanced (Toggle Plain Text) #include <iostream> using namespace std; void func1(); int count; // this is a global variable int main() { int i; // this is a local variable for(i=0; i<10; i++) { count = i * 2; func1(); } return 0; } void func1() { cout << "count: " << count; // access global count cout << '\n'; // output a newline } #include <iostream> using namespace std; void func1(); int count; // this is a global variable int main() { int i; // this is a local variable for(i=0; i<10; i++) { count = i * 2; func1(); } return 0; } void func1() { cout << "count: " << count; // access global count cout << '\n'; // output a newline }

May 19th, 2005, 1:31 PM	#2
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	That will work fine, but yes, you're right - you can't declare a local variable if you have a global variable with the same name. __________________ Me :: You :: Them

May 19th, 2005, 2:06 PM	#3
Ricky205 Newbie Join Date: May 2005 Posts: 4 Rep Power: 0	ok thanks , now I got a second question about it .. what if u were to declare a global variable named var for instance. Now lets say you have 2 functions .. can u use var in both functions with each function having a different value for it .. say : (Toggle Plain Text) outside all functions: int var; inside the first function : var = 10; inside the second function : var = 50; outside all functions: int var; inside the first function : var = 10; inside the second function : var = 50;

May 19th, 2005, 4:33 PM	#4
mackenga Professional Programmer Join Date: Mar 2005 Location: Glasgow, Scotland Posts: 317 Rep Power: 4	Yes, variables in different functions don't clash with each other. If you have a global variable of a given name, for example foo, you can create a local one with the same name if you please. (Toggle Plain Text) int foo; int bar() { int foo; } int foo; int bar() { int foo; } Code inside the function, that is in the same scope as the local variable foo, will refer to the local variable when it refers to the variable called foo. Code outside it, in other functions, will refer to the global foo, unless they define a variable called foo of their own. -- I just realised I kind of missed the point of what you were asking there. If you declared the global variable var, you could use it in two different functions, and they'd both be referring to the same variable. So in func1, if you set var to 10, then call func2, func2 will see the value 10 in it. If it now sets it to 20 and returns, and you then call func1, func1 sees the new value. A global variable is one name and one storage location and it's shared throughout the program. It can only take one value at a time, and any function that assigns a value to the global variable will change the value any other function sees. Last edited by mackenga; May 19th, 2005 at 4:35 PM.

May 20th, 2005, 3:31 AM	#5
Berto Programming Guru Join Date: Aug 2004 Posts: 1,022 Rep Power: 6	I am not sure about c++ but as a rule of thumb when i am using java i tend to use this.<variable> for gloadls and then just the name of the variable for the local one. __________________ "Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT'S relativity." - Albert Einstein

May 20th, 2005, 3:14 PM	#6
mackenga Professional Programmer Join Date: Mar 2005 Location: Glasgow, Scotland Posts: 317 Rep Power: 4	There aren't any global variables in Java. As well as not being inside a function, global variables (in languages that have them) are not inside anything else.

May 21st, 2005, 10:25 AM	#7
Berto Programming Guru Join Date: Aug 2004 Posts: 1,022 Rep Power: 6	well its close enough to a global variable. __________________ "Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT'S relativity." - Albert Einstein

May 21st, 2005, 11:12 AM	#8
Aphex_Twin Newbie Join Date: Mar 2005 Posts: 27 Rep Power: 0	You can use the same name over and over in functions and blocks. C/C++ has a means of hiding the hierarchically higher variables. For instance: (Toggle Plain Text) int a; void f () { int a; { int a; { int a; } } } int a; void f () { int a; { int a; { int a; } } } Is valid code. We have four diferent instances of the same variables (or four diferent variables with the same name). In each case the program sees only the most recently declared version. The global variable is accessible by the resolution operator (i.e. ::a). Though you cannot acces hierarchically higher local variables (such as the second "a" from the third block). Global versions of variables can be accessed through ::my_var where my_var is the name of the variable.

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Programming Forums > Application Development > C++

C++ Global Variables question